German SSTO VTOVL orbital launch vehicle. In 1969 rocket pioneer Dietrich Koelle was working at MBB (Messerschmitt-Bolkow-Blohm). There he sketched out a reusable VTOVL design called BETA using Bono's SASSTO as a starting point. The vehicle, taking European technology into account, was a bit heavier than Bono's design. But the thorough analysis showed even this design would be capable of delivering 2 tonnes of payload to orbit.

BETA ("Ballistisches Einstufiges Träger-Aggregat" - Ballistic Single-stage Carrier Vehicle) represented a combination of features of existing launch vehicles, the Apollo Capsule and the LM Lunar Lander: re-entry technique and technology as in case of Apollo, final landing approach and touchdown as in case of the lunar module. BETA was an unmanned, fully automatic shuttle system. A crew could be added if desired for certain orbital operations. The following features characterized the BETA concept:

A short conical body (small length/diameter ratio, low centre of gravity) with heat-shield for re-entry

Use of the heat-shield as a plug-nozzle for performance increase

A propulsion system consisting of 12 or more single high-pressure LH2/ LOX engines arranged around the central plug-nozzle (heat-shield)

6 retractable legs for the final vertical landing phase.

BETA had a single large hydrogen tank and a toroidal (multi-cell) LOX tank in a conical arrangement. This resulted in a low c.g. for good stability in flight and on the ground. The large heat shield served during ascent as a plug-nozzle, providing performance improvement for the 12 high-pressure, conventional configuration, low thrust level rocket motors. The motors were rigidly mounted around the LOX tank. No gimbaling was required since thrust vector control was to be achieved by single engine throttling. The relatively large bottom area resulted in small aerothermodynamic loads (1/3 of Apollo) during re-entry and high engine performance during ascent. Final landing was to be performed by retro-thrust (4 motors only) and six retractable legs. The payload was mounted atop of the vehicle with or without a special fairing. This arrangement meant much less geometrical restrictions than a cargo compartment within a winged shuttle.

Mass breakdown for the BETA I with 115 t nominal propellant mass was as follows:

The principal advantage of the single-stage BETA-Vehicle was the possibility to launch it from any location in Europe, since no stages or parts fell away during ascent. Vertical take-off and vertical landing in combination with the landing leg system allowed continuous abort capability in the critical launch phase. It also allowed an outstanding means of test operations - incremental advances from ground testing to flight tests.

The ascent trajectory optimisation of a single-stage vehicle of the BETA type was more complex than for a multistage vehicle. The vehicle performance was much more sensitive to thrust optimisation taking into account the atmospheric and dynamic parameters. The results showed that the launch acceleration should be higher than that for conventional vehicles: 1.5 g. Thereafter a thrust programme had to be applied which decreased the thrust level down to 10% in such a way that the optimum ascent time was achieved. This was found to be 500 sec for a 200 km orbit, with the acceleration never exceeding 3.5 g. The propellant mixture ratio was also involved in the optimisation: it was shifted from 5.5 at launch to 8.0 before cutoff, taking into account the large difference in expansion ratio.

BETA would remain in orbit as required by operations and flight mechanics before a braking impulse (given by 4 engines) initiated return to Earth. A secondary propulsion system for orbital manoeuvres, attitude control and propellant orientation aligned the vehicle before the manoeuvre and allowed control during descent. The plug-nozzle heat shield protected the vehicle during the critical re-entry phase, while the engine nozzles were cooled by hydrogen circulation. Finally, a constant descent velocity of about 70 m/sec was reached which was then reduced to zero by a final brake impulse given by 4 engines. The legs absorbed the final landing shock.

The LM Lunar Lander had demonstrated point-landing capability on unprepared terrain and this method should be feasible on Earth too. The trajectory had to be corrected such that BETA reached a point 5 to 10 km above its landing area and performed practically a vertical descent - it had only a limited cross-range capability.

The vacuum performance of a single high-pressure rocket engine with a nozzle area ratio of about 250 was about 460 sec specific impulse, as indicated in contemporary design studies of Rocketdyne, Aerojet and Pratt & Whitney for the US Space Shuttle engine. MBB had demonstrated high-pressure engines with this chamber pressure in the United States at the Reno Test Site. The topping cycle principle applied for these engines was demonstrated by MBB with a 6 tonne LOX/Kerosene engine.

The problem of rocket engines with a conventional (bell) nozzle was the fact that they could only be optimised for one altitude, which meant losses in all other flight regions. A larger nozzle with a higher expansion ratio provided better specific impulse in space but much lower at launch. It also meant larger and heavier engines to achieve the required launch acceleration. The plug nozzle was a solution to this problem since the expansion was self-adapting to the external pressure and was believed to always deliver optimum specific impulse. However the development of a plug-nozzle engine with 10 m diameter and several hundred tonnes thrust represented a relatively large development effort, even if the segmented design concept was used.

As a compromise, the BETA concept assumed a series of 12 to 20 single rocket engines with a small thrust level compared to the vehicle mass, grouped around a central plug that also served as the heat shield during re-entry. In the launch phase this resulted in an effective area ratio 35:1 and a specific impulse of 380 sec. Above 10 km the plug nozzle became effective, finally attaining an effective expansion ratio of 500:1 and a specific impulse of 470 seconds at 100 km altitude. The average specific impulse using the selected ascent profile was 458 sec.

The additional velocity required for recovery of the BETA was relatively small - 3% of the total amount. Therefore, the penalty for recovery was not very large, taking into account the fact that the heat-shield was used as nozzle structure. The velocity requirements were calculated as follows:

The flight test facilities and operations required for the BETA Shuttle Concept were certainly considerably less complex and less costly than for a two-stage winged system. Only one system had to be developed and no separation problems had to be solved. Flight tests could evolve out of ground testing - the critical phase of take-off could be tested repeatedly - a unique feature compared with any other concept. The major development effort would lie in the engine system (not the single engine): the optimisation of the plug nozzle and engine integration. Another problem was the heat shield, which had to be lightweight and survive more than one flight to reduce the refurbishment cost. Ablative and hydrogen-cooled metal shields would have to be investigated. The specific transportation cost would be influenced by the vehicle size and resulting payload size; the refurbishment cost; the number of launches per year, and the possible number of re-uses for the same vehicle. A preliminary analysis for the small BETA I vehicle led to a specific transportation cost of some 300 $/kg.

VTOVL The concept of a reusable single-stage-to-orbit Vertical Take-Off Vertical Landing (VTOVL) launch vehicle that would reenter and return to its launch site for turnaround and relaunch was first proposed by Philip Bono in the 1960's. The appealing simplicity of the concept has been offset by the technological risk in developing it. The problem with any single-stage-to-orbit concept is that if the empty weight of the final vehicle has been underestimated it will not be able to deliver any payload to orbit, or even reach orbit. Since weight growth of up to 20% is not unknown in aerospace projects, this is a very real threat which has made both NASA and private investors reluctant to invest the billions of dollars it would take to develop a full-scale flight vehicle. More...